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Effect of InP substrate thermal degradation on MBE InGaAs layers
Journal of Crystal Growth 69 (1984) 635—638 North-Holland, Amsterdam
635
LETTER TO THE EDITORS EFFECT OF InP SUBSTRATE THERMAL DEGRADATION ON MBE InGaAs LAYERS Fernando GENOVA, Claudio PAPUZZA, Cesare RIGO and Sandro STANO Sezione Informazione e Documentazione, CSELT Turin, Via G. Reiss Romoli 274, 1-10148 Torino, Italy
Received 3 July 1984; manuscript received in final form 17 September 1984
A common cleaning technique of lnP substrates is the thermal in situ stabilization under As fluxes at the oxide desorption temperature. Since this desorption temperature is well above the upper limit of congruent sublimation, preferential desorption of phosphorus occurs with the formation of indium droplets. In the presence of arsenic flux these droplets precipitate as InAs crystallite with pyramidal habit which can be at the origin of the oval and whisker defects commonly reported in previous works.
1. Introduction The growth of high quality materials with the MBE (molecular beam epitaxy) technique is strongly dependent on the status of the substrate surface. So a careful control of the cleaning procedure outside and inside the deposition chamber is required. To achieve contaminant- and oxide-free surfaces on InP a thermal stabilization under As pressure is commonly used. Since the oxide desorption temperature is in the range of InP thermal degradation, the behavior of (100) substrates as a function of the temperature was studied, either under a stabilizing As flux or under UHV conditions, in order to define the optimal operative parameters.
nants on InP samples inside the MBE, is a stabilization under As flux at a temperature well above the upper limit of congruent desorption [5J. This flux is necessary to avoid the thermal degradation of InP, at the temperature required for the cornplete desorption of oxide species, evaluated at about 490°C [6]. The temperature is measured with a thermocouple when the oxygen Auger peak disappears. We have observed that this temperature is slightly dependent on the As pressure employed. At very high As pressure (~A~> 1 x lO~ Torr) little amount of oxygen is found on the Auger spectra even at temperatures higher than 520°C, whereas under normal conditions (~A~= 2 X 120~ Torr), we obtain at 490°C an oxide free surface within the limit of the Auger sensitivity.
2. Experimental 3. Thermal degradation of InP The system used in this work was a computer controlled Varian 360 MBE. The (100) oriented InP substrates were all submitted to the same chemical cleaning procedure [1] which leaves the minimum amount of carbon contamination among the various cited in the literature for InP [2—4].It
The degradation of InP is a threshold phenomena due to the preferential desorption of phosphorus from the surface, leading to the formation of In droplets at the surface. Fig. 1 shows partial pressures of In, P2 and P4 as
has been reported several times that one of the most important parameters for MBE high quality materials is the substrate cleanliness. A normal procedure for the removal of residual contami-
monitored by a computer controlled quadrupole mass spectrometer either under UHV conditions or under As overpressures of 2 X iO~ Torr. It is noteworthy that in the range of higher tempera-
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
636
F. Genova et a!.
/
Effect of InP substrate thermal degradation of MBE InGaAs
a
io.7 62 P 2 8 io
~:
124 P4~
~ io-12
Fig. 2. Indium droplet on substrate heated at 450°C under UHV conditions for 5 mm.
10—13 11
I~ 1 2
I. 1 3 ~
1 4
1 5
[K~I
io-6
b
tures the vaporization energies of fig. Ia are consistent with the literature data [7]. Under As stabilization conditions, the minimum temperature for thermal pit formation is increased by about 40°C in comparison to the UHV conditions. Nevertheless, at temperatures normally used for the oxide removal, the substrate is in the range of thermal degradation (fig. lb)
62 P
even with As stabilization.
2
io~
A droplet formed on a substrate held at 450°C under UHV for 5 mm is shown in fig. 2. Their diameter range from 1 ~tm to less2.than 100 nm, with a density from iO~to iO~cm
-
8
124P
io
4
1O~l0 . 115 In 10-li
-
\
\
I ~ ‘
12 io
1.1
I
1.2
1.3
1.4
1.5
T Fig. I. Partial pressures of substrate desorbing species, as monitored by QMS under: (a) UHV conditions, (b) As flux,
~l—
I
Fig. 3. Effect of RED beam on the thermal degradation of a substrate.
F Genova et a!.
/
637
Effect of InP substrate thermal degradation of MBE InGaAs
We observed that the use of the reflected electron diffractometer (RED) during the heating has a marked effect on the InP decomposition. Where the electron beam hit the surface no thermal damage occurred as shown in fig. 3. Indium droplets in the presence of stabilizing As change their morphology to regular pyramidal crystallites (fig. 4), formed mostly of InAs as detected by XMA (X-ray micro-analysis). Phase diagrams for the In—As system show for metallic arsenic in liquid indium a solubility of 10% at a O 1 ~~tm I Fig 4. InAs cr~stallitedue to stabilization under As flux of In droplet.
temperature of 450°Cwhich gives an equilibrium vapor pressure of 2 x 10~Torr. At ‘~As 2 x iO~ Torr,’ liquid indium is supersaturated and the precipitation of InAs crystallites can occur. Fig. 5 refers to crystallites grown at 45°Cwith an As pressure of 6 >< I06 Torr, and shows the coexistence of droplets and forming pyramids. We suppose that these droplets can explain the formation of morphological features on MBE layers. Indium droplets can be the first step for the growth of whisker defects (through a VLS mechanism) or the nucleation sites of oval defects commonly reported in previous works [8,9]. A one-toone correspondence between droplets and morphological defects was found on InGaAs and AlInAs layers grown on thermally damaged substrates as shown in fig. 6. =
1 pm Fig. 5. Forming InAs crystallites coexisting with In droplets.
20i.zm 111111.
II
II
111111111
III•
1111111
II~’IIIIIIIIIII~
II”
2O~.tm I!!I••III~’~~
Fig. 6. Correspondence between the thermal damage of the substrate (a) and growth defects (h).
I
I
638
F. Genova et a!.
/
Effect of InP substrate thermal degradation of MBE InGaAs
4. Conclusions
[2] Y. Nishitani and T. Kotani, J. Electrochem. Soc. 126 (1979) 2269.
Thermal treatment of InP substrates for the removal of surface oxides, can lead to the formation of In microdroplets that under As stabilization flux, transform in pyramidal InAs crystallites. We think that these droplets can be one of the causes for the origin of the oval and whisker defects commonly reported on MBE layers.
[3] D.T. Clark, T. Fok, G.G. Roberts and R.W. Sykes, Thin Solid Films 70 (1980) 261. [4] PA. Bertrand, J. Vacuum Sci. Technol. 18 (1981) 28. [5] G.J. Davies, R. Heckingbottom, H. Ohno, C.E.C. Wood and AR. Calawa, AppI. Phys. Letters 37 (1980) 290. [6] C.E.C. Wood, K. Singer and T. Ohashi, J. AppI. Phys. 54 (1983) 2732. [7] RE. Honig and D.A. Kramer, RCA Rev. 30 (1969) 285. [81H. Saito, JO. Borland, H. Asahi, H. Nagai and K. Nawata, J. Crystal Growth 64 (1983) 521.
References [1] KY. Cheng, A.Y. Cho, W.R. Wagner and W.A. Bonner, J. AppI. Phys. 52 (1981) 1015.
635
LETTER TO THE EDITORS EFFECT OF InP SUBSTRATE THERMAL DEGRADATION ON MBE InGaAs LAYERS Fernando GENOVA, Claudio PAPUZZA, Cesare RIGO and Sandro STANO Sezione Informazione e Documentazione, CSELT Turin, Via G. Reiss Romoli 274, 1-10148 Torino, Italy
Received 3 July 1984; manuscript received in final form 17 September 1984
A common cleaning technique of lnP substrates is the thermal in situ stabilization under As fluxes at the oxide desorption temperature. Since this desorption temperature is well above the upper limit of congruent sublimation, preferential desorption of phosphorus occurs with the formation of indium droplets. In the presence of arsenic flux these droplets precipitate as InAs crystallite with pyramidal habit which can be at the origin of the oval and whisker defects commonly reported in previous works.
1. Introduction The growth of high quality materials with the MBE (molecular beam epitaxy) technique is strongly dependent on the status of the substrate surface. So a careful control of the cleaning procedure outside and inside the deposition chamber is required. To achieve contaminant- and oxide-free surfaces on InP a thermal stabilization under As pressure is commonly used. Since the oxide desorption temperature is in the range of InP thermal degradation, the behavior of (100) substrates as a function of the temperature was studied, either under a stabilizing As flux or under UHV conditions, in order to define the optimal operative parameters.
nants on InP samples inside the MBE, is a stabilization under As flux at a temperature well above the upper limit of congruent desorption [5J. This flux is necessary to avoid the thermal degradation of InP, at the temperature required for the cornplete desorption of oxide species, evaluated at about 490°C [6]. The temperature is measured with a thermocouple when the oxygen Auger peak disappears. We have observed that this temperature is slightly dependent on the As pressure employed. At very high As pressure (~A~> 1 x lO~ Torr) little amount of oxygen is found on the Auger spectra even at temperatures higher than 520°C, whereas under normal conditions (~A~= 2 X 120~ Torr), we obtain at 490°C an oxide free surface within the limit of the Auger sensitivity.
2. Experimental 3. Thermal degradation of InP The system used in this work was a computer controlled Varian 360 MBE. The (100) oriented InP substrates were all submitted to the same chemical cleaning procedure [1] which leaves the minimum amount of carbon contamination among the various cited in the literature for InP [2—4].It
The degradation of InP is a threshold phenomena due to the preferential desorption of phosphorus from the surface, leading to the formation of In droplets at the surface. Fig. 1 shows partial pressures of In, P2 and P4 as
has been reported several times that one of the most important parameters for MBE high quality materials is the substrate cleanliness. A normal procedure for the removal of residual contami-
monitored by a computer controlled quadrupole mass spectrometer either under UHV conditions or under As overpressures of 2 X iO~ Torr. It is noteworthy that in the range of higher tempera-
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
636
F. Genova et a!.
/
Effect of InP substrate thermal degradation of MBE InGaAs
a
io.7 62 P 2 8 io
~:
124 P4~
~ io-12
Fig. 2. Indium droplet on substrate heated at 450°C under UHV conditions for 5 mm.
10—13 11
I~ 1 2
I. 1 3 ~
1 4
1 5
[K~I
io-6
b
tures the vaporization energies of fig. Ia are consistent with the literature data [7]. Under As stabilization conditions, the minimum temperature for thermal pit formation is increased by about 40°C in comparison to the UHV conditions. Nevertheless, at temperatures normally used for the oxide removal, the substrate is in the range of thermal degradation (fig. lb)
62 P
even with As stabilization.
2
io~
A droplet formed on a substrate held at 450°C under UHV for 5 mm is shown in fig. 2. Their diameter range from 1 ~tm to less2.than 100 nm, with a density from iO~to iO~cm
-
8
124P
io
4
1O~l0 . 115 In 10-li
-
\
\
I ~ ‘
12 io
1.1
I
1.2
1.3
1.4
1.5
T Fig. I. Partial pressures of substrate desorbing species, as monitored by QMS under: (a) UHV conditions, (b) As flux,
~l—
I
Fig. 3. Effect of RED beam on the thermal degradation of a substrate.
F Genova et a!.
/
637
Effect of InP substrate thermal degradation of MBE InGaAs
We observed that the use of the reflected electron diffractometer (RED) during the heating has a marked effect on the InP decomposition. Where the electron beam hit the surface no thermal damage occurred as shown in fig. 3. Indium droplets in the presence of stabilizing As change their morphology to regular pyramidal crystallites (fig. 4), formed mostly of InAs as detected by XMA (X-ray micro-analysis). Phase diagrams for the In—As system show for metallic arsenic in liquid indium a solubility of 10% at a O 1 ~~tm I Fig 4. InAs cr~stallitedue to stabilization under As flux of In droplet.
temperature of 450°Cwhich gives an equilibrium vapor pressure of 2 x 10~Torr. At ‘~As 2 x iO~ Torr,’ liquid indium is supersaturated and the precipitation of InAs crystallites can occur. Fig. 5 refers to crystallites grown at 45°Cwith an As pressure of 6 >< I06 Torr, and shows the coexistence of droplets and forming pyramids. We suppose that these droplets can explain the formation of morphological features on MBE layers. Indium droplets can be the first step for the growth of whisker defects (through a VLS mechanism) or the nucleation sites of oval defects commonly reported in previous works [8,9]. A one-toone correspondence between droplets and morphological defects was found on InGaAs and AlInAs layers grown on thermally damaged substrates as shown in fig. 6. =
1 pm Fig. 5. Forming InAs crystallites coexisting with In droplets.
20i.zm 111111.
II
II
111111111
III•
1111111
II~’IIIIIIIIIII~
II”
2O~.tm I!!I••III~’~~
Fig. 6. Correspondence between the thermal damage of the substrate (a) and growth defects (h).
I
I
638
F. Genova et a!.
/
Effect of InP substrate thermal degradation of MBE InGaAs
4. Conclusions
[2] Y. Nishitani and T. Kotani, J. Electrochem. Soc. 126 (1979) 2269.
Thermal treatment of InP substrates for the removal of surface oxides, can lead to the formation of In microdroplets that under As stabilization flux, transform in pyramidal InAs crystallites. We think that these droplets can be one of the causes for the origin of the oval and whisker defects commonly reported on MBE layers.
[3] D.T. Clark, T. Fok, G.G. Roberts and R.W. Sykes, Thin Solid Films 70 (1980) 261. [4] PA. Bertrand, J. Vacuum Sci. Technol. 18 (1981) 28. [5] G.J. Davies, R. Heckingbottom, H. Ohno, C.E.C. Wood and AR. Calawa, AppI. Phys. Letters 37 (1980) 290. [6] C.E.C. Wood, K. Singer and T. Ohashi, J. AppI. Phys. 54 (1983) 2732. [7] RE. Honig and D.A. Kramer, RCA Rev. 30 (1969) 285. [81H. Saito, JO. Borland, H. Asahi, H. Nagai and K. Nawata, J. Crystal Growth 64 (1983) 521.
References [1] KY. Cheng, A.Y. Cho, W.R. Wagner and W.A. Bonner, J. AppI. Phys. 52 (1981) 1015.